The present invention relates to an umbilical or power cable that is used in an offshore application such as oil and gas field developments, subsea pumping, processing and renewable energy installations.
An umbilical consists of a group of one or more types of elongated active umbilical elements, such as electrical cables, optical fiber cables, steel pipes and/or hoses, cabled together for flexibility, over-sheathed and, when applicable, armored for mechanical strength. Umbilicals are typically used for transmitting power, signals and fluids (for example for fluid injection, hydraulic power, gas release, etc.) to and from a subsea installation.
The umbilical cross-section is generally circular. The elongated elements of the umbilical are wound together either in a helical or in a S/Z pattern. In order to fill the interstitial voids between the various umbilical elements and obtain the desired configuration for the umbilical, filler components may be included within the voids.
ISO 13628-5/API (American Petroleum Institute) 17E “Specification for Subsea Umbilicals”, provides standards for the design and manufacture of such umbilicals.
Subsea umbilicals are installed at increasing water depths, commonly deeper than 2000 m. Such umbilicals have to be able to withstand severe loading conditions during their installation and their service life.
The main load bearing components in use for withstanding the axial loads due to the weight and to the movements of the umbilical are steel pipes (see U.S. Pat. No. 6,472,614, WO93/17176 and GB2316990), steel rods (see U.S. Pat. No. 6,472,614), composite rods (see WO2005/124213), tensile armor layers (see FIG. 1 of U.S. Pat. No. 6,472,614), or steel wire ropes.
The other elements of the umbilical, i.e. the electrical and optical cables, the thermoplastic hoses, the polymeric external sheath and the polymeric filler components do not contribute significantly to the tensile strength of the umbilical.
Electrical cables used in subsea umbilicals fall into two distinct categories respectively known as power cables and signal cables. Power cables are used for transmitting high electrical power (typically a few MW) to powerful subsea equipments such as pumps. Power cables are rated at voltages comprised between 1 kV and 200 kV, and generally between 6 kV and 35 kV (medium voltage range).
Signal cables are generally used for transmitting signals and low power (<1 kW) to electrical devices on the seabed. Signal cables are generally rated at a voltage smaller than 3000V, and typically smaller than 1000V.
An umbilical comprising at least one power cable is called a power umbilical. Thus, a power umbilical includes one or more electrical power cables formed from one or more conductors.
The conductors of these power cables within the subsea power umbilical are copper as specified in ISO 13628-5. They are not load bearing components, because of the low tensile strength and high specific gravity of copper. These heavy copper conductors add considerable weight to the umbilical and have very poor load carrying capacity, thus limiting the sea depth that the umbilical can be deployed at. Unless protected, these electrical conductors may be damaged by excessive elongation or crushing especially under severe conditions such as in deep water and/or dynamic umbilicals.
An object of the present invention is to solve this problem and provide a power umbilical in dynamic or deep water applications.